Electrolyte migration control for large area MCFC stacks

ABSTRACT

The present invention relates to a new strategy against the electrolyte migration through the gasket in externally manifolded MCFC stacks. This is obtained by endowing a stack of molten carbonate fuel cells (MCFCs) of an electrolyte management tool separated from the cells by an electronically conductive material which is impervious to gas, characterized by the combination of the following elements:—a positive reservoir component external to the cathode of the first cell on the positive side of the battery, wherein said reservoir consists of one ore more porous layers of electronically conductive material and comprises at least one gas distributor, and—a negative reservoir component external to the anode of the last cell on the negative side of the battery, wherein said reservoir consists of one ore more porous layers of electronically conductive material.

FIELD OF THE INVENTION

The present invention relates to a Molten Carbonates Fuel Cell apparatuswhich allows to achieve an active control of the electrolyte migrationthrough the gasket in externally manifolded stacks. The aim is tominimise the impact of this effect on performance, when the stack isconstituted by a large number (≧50) of large area (≧3500 cm²) cells.Such a strategy does not require modifications to particular cells, butall the cells can be made by the same components.

BACKGROUND OF THE INVENTION

A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) andoxygen to create electricity by an electrochemical process.

A single fuel cell consists of an electrolyte sandwiched between twothin electrodes (a porous anode and cathode). There are different fuelcell types, for all hydrogen, or a hydrogen-rich fuel, is fed to theanode, oxygen, or air, to the cathode.

For instance, for polymer exchange membrane (PEM) and phosphoric acidfuel cells, protons, generated by the anode reaction, move through theelectrolyte to the cathode to combine with oxygen and electrons,producing water and heat.

For alkaline, molten carbonate, and solid oxide fuel cells, negativeions travel through the electrolyte to the anode where they combine withhydrogen to generate water and electrons. The electrons from the anodeside of the cell cannot pass through the membrane to the positivelycharged cathode; they must travel around it via an electrical circuit toreach the other side of the cell. This movement of electrons is anelectrical current.

Molten Carbonate Fuel Cells (MCFC) are in the class of high-temperaturefuel cells. The higher operating temperature allows them to use naturalgas directly without the need for a fuel processor and have also beenused with low-Btu fuel gas from industrial processes other sources andfuels. Developed in the mid 1960s, improvements have been made infabrication methods, performance and endurance.

MCFCs work quite differently from other fuel cells. These cells use anelectrolyte composed of a molten mixture of carbonate salts. Twomixtures are currently used: lithium carbonate and potassium carbonate,or lithium carbonate and sodium carbonate. To melt the carbonate saltsand achieve high ion mobility through the electrolyte, MCFCs operate athigh temperatures (650° C.).

When heated to a temperature of around 650° C., these salts melt andbecome conductive to carbonate ions (CO₃ ²⁻). In operation, these ionsare generated by the cathode reaction and flow from the cathode to theanode where they combine with hydrogen to give water, carbon dioxide andelectrons. These electrons are routed through an external circuit backto the cathode, generating electricity and by-product heat.

The reactions which take place are the following:Anode Reaction: CO₃ ²⁻+H₂=>H₂O+CO₂+2e ⁻Cathode Reaction: CO₂+½O₂+2e ⁻=>CO₃ ²⁻Overall Cell Reaction: H₂(g)+½O₂(g)+CO₂ (cathode)=>H₂O (g)+CO₂ (anode)

The higher operating temperature of MCFCs has both advantages anddisadvantages compared to the lower temperature PAFC and PEFC. At thehigher operating temperature, CO is a fuel and not a poison, withformidable benefit in terms both of fuel source acceptability and ofmethane reforming. Moreover, with regard to the reforming of naturalgas, it can occur by using directly as thermal source the waste heatitself of the MCFC. Additional advantages include the ability to usestandard materials for construction, such as stainless steel sheet, andallow use of nickel-based catalysts on the electrodes. The by-productheat from an MCFC can be used to generate high-pressure steam that canbe used in many industrial and commercial applications.

With regard to the main functional requirements, the complete filling ofevery interstice in the porous ceramic component inserted between anodeand cathode (electrolyte matrix) is a key factor. This not onlyconstitutes a barrier for the gases but also prevents the sinking ofperformance (in fact, in the vacancies of electrolyte the transport tothe anode of the CO₃ ²⁻ ion generated by the cathode does not occur).For both the electrodes, an essential characteristic for the completionof the reactions is the constant presence of a triphasic contact surfacegas/C₃ ²⁻ ion/electron. It is then evident that besides theelectro-catalytic properties of the electrode material, the performancedepends on the access/removal from the reaction sites of gas and CO₃ ²⁻ion. The performance is therefore unacceptable as well when theelectrodes are “flooded” as when the electrolyte quantity isinsufficient.

For each of the two electrodes there is an optimal filling degree forenhancing the performances; moving away from it means diminishing theefficiency degree. There is anyway a range in which such degree is stillacceptable and the cells characteristics can be in any case exploited.

In working condition, since the carbonates amount gradually decreases,the filling degree changes with the time. In order to maintain thefilling levels higher then the minimum required, it is necessary tostart from filling levels which are higher than the optimal ones.

The carbonates distribute spontaneously in the pores with highercapillary retention properties.

In order to control the electrolyte repartition among components, it isnecessary to choose the correct volumes in the design of the cell andrespect a rigorous hierarchy on the base of their retention properties.

Since it is necessary that the matrix remains in any case full, thediversion of the filling degree must regard exclusively the electrodes.The reaction at the anode is less penalized than the one which takesplace at the cathode by the filling degrees which are far away from theoptimal values. Hence, the design of the cell tends therefore to use theanode by concentrating on it the initial surplus of electrolyte andevacuating it for first. The configuration of a single cell consists ofa 3-layers “sandwich” (anode, matrix of electrolyte, cathode) placedbetween metallic pieces intended for the distribution of gas and for thetransport of current. In practice, a molten carbonate fuel cell (whichconventionally is indicated with the word “stack”) is a modularstructure constituted by many elementary cells electrically connected inseries but with parallel gas inlets. The electrical connection betweenthe cells is achieved with a metallic separator (electronic conductor)between the anode compartment of the one cell and the cathodecompartment of the adjacent one. At each end of the cell there is ametallic plate; they are normally indicated as “end plates”, one at theanode and the other at the cathode.

In order to provide the single cells with gas, the commonly used waysare two: external manifold and internal manifold. In the first case eachone of the four lateral faces has a specific function: fuel inlet, fueloutlet, oxidation agent inlet, oxidation agent outlet. The first and thesecond couple of faces are in opposite positions to each other.Normally, on every face a manifold is placed directly against the faceof the cells pack. From the internal area to the manifold the gasreaches directly the parts to which it is designated.

In the internal manifold, the inlet conducts of the gas to the singlecells are obtained through complex grooves in the bipolar plates whichdivide one cell from the other.

STATE OF THE ART

The state of the art has been till now confronted with the problemsconnected to the use of an external manifold of the electrolytemigration through the gasket of the manifold. With the externalmanifold, the necessary tightness for maintaining the gas separated fromthe external environment requires the use of a gasket on the perimeterof the cell pack surface which is in contact with the manifold.

For technical reasons the use of a metallic o-ring is not possible: theactual solutions regard the use of different materials, like forexample, tissues and caulks which normally are mixtures of ceramicpowders with the add of fluidizing means. In any case, the result is aporous structure permeable to the electrolyte and in communication withthe electrolyte of all the cells. As a consequence, the single cellscould not be considered independent, since the gaskets of the manifoldrepresent possible passages of electrolyte from one cell to another.

In the stack the cells are electronically connected in series:consequently there is a potential gradient along the stack between thetwo ends. This causes a migration of the alkaline ions of electrolytethrough the gaskets from the cells which are on the positive extremityto the cells connected to the negative one, with subsequent “overproduction” of the CO₃ ²⁻ ions in the “negative” cells and with acorresponding “depletion” in the “positive” cells. The cells which areon the positive extremity of the stack show a loss of electrolyte, inaddition to the loss which is caused by consumption mechanisms.

By changing the gasket characteristics, even the electrolyte quantitywhich is transferred from the positive to the negative side of the stackis different. This quality does not depend on the number of cellscontained in the stack: the migration involves only the cells at the twoextremities and it does not interest the internal ones.

The consequences for the cells at the positive extremity are always thesame: the anticipated emptying causes a performance fall and, becausethe electrolyte quantity in the electrodes becomes insufficient, whenthe emptying process reaches a critical level the electrolyte matrixloses its tightness properties to the gases and the mixing of fuel andoxidant takes place directly into the cell, with the consequent quickdegradation of hardware.

Depending on the cells surface, on its internal structure and on theconfiguration of the gasket, the time which is necessary to reachcritical conditions changes but the dynamic of the mechanisms is alwaysthe one cited above. For the cells at the negative side the risk is aflooding of the electrodes with a significant fall of performance. Thismechanism extends then to a higher and higher number of cells dependingon the flooding level of the extremity cells. Furthermore, unlessparticular compositions are used, the mobility of the alkaline ions inthe electrolyte is different (like for example the one of the ions Li⁺and K⁺ for the binary eutectic at 62 mol % Li and 38 mol % K, which hasalways been the reference composition).

Since this causes that the migration speeds by the gaskets aredifferent, at the negative side the electrolyte shows a higherconcentration of the most mobile ion (K⁺ in the before cited example),and at the positive side of the less mobile ion (Li⁺ in the example).This phenomenon alters further the cells performance at the twoextremities and, in particular cases, it could also lead to thesolidification of the electrolyte at the working temperature (650° C.).

For the cells at the negative side of the stack, the dynamic of themechanisms changes radically in accordance to different factors, inparticular the surface of the cell, its internal structure, thecomposition of the electrolyte in the different cells and theconfiguration of the gaskets.

In order to avoid the consequence of the migration of electrolyte inmolten carbonate fuel cells with external manifold, a known solutionused in the state of the art is to differentiate the internal structureof some cells which are connected to the positive side of the stack, andof other cells which are connected the negative side.

Accordingly, the solution regards firstly the use of thicker electrodesso that the electrolyte disposes, inside of them, of a greater volume incomparison to the one of the electrodes in ordinary cells which are“internal” to the stack and secondly in a different load of electrolytein the “more external” cells, having a higher quantity of it in thecells at the positive side.

In any case, considering the migration phenomenon even by using the bestgaskets, in order to fabricate stacks which should last 40,000 it wouldbe necessary to use in the “more external” cells electrodes which shouldbe 8 to 10 times thicker than the ones in the ordinary cells.

With an internal structure which is so different from the optimal one,the functionality of such “reservoir cells” is so compromised that inparticular operating conditions could also consume energy instead ofproducing it. Furthermore the presence of non-standard components has anegative influx on the fabrication costs: in particular, it is necessaryto print and to treat separately the separator plates.

Additionally, in the start-up cycle of the stack the presence of“reservoir cells” slows down the entire process, with consequenttechnological and economical disadvantages.

Another approach tends to limit the migration effect through the controlof particular properties of the gasket. This is for example the case ofthe U.S. Pat. No. 5,110,692 of Farooque et al. The solution consists inthe introduction of different barriers in the gasket parts which areinvolved in the migration process. Those barriers should slow theelectrolyte flux. The application of such a solution involves high costsand is also quite difficult to apply: it has also a very weakeffectiveness which is affected by movements of the parts very difficultto be controlled during the working process.

Another approach consists in limiting the electrolyte transfer from thecells to the gaskets by modifying the porous characteristics of thecomponents in the cells corner range, where the contact with the gasketsoccurs. In this zone it is necessary to create dry ranges of electrolytewhich have to be organised in “labyrinths” because the separationbetween the anodic and the cathodic gases must be in any casemaintained.

An example of this kind of solution is disclosed in the U.S. Pat. No.4,659,635 by Raiser et al. and in the U.S. Pat. No. 5,478,663 byCipollini et al. The above-mentioned modifications should beincorporated in all the cells and not only in the external ones (i.e.the ones which normally are involved in the migration process). This isbecause otherwise, on the side of the positive pole, the first cells tobe evacuated would be the non-protected ones instead of the protectedones and on the contrary at the negative side the first cells to beflooded would be the non-protected ones instead of the protected ones.Those solutions appear to be too expensive to be applied.

Consequently, all the cited solutions are characterised by a weakcommercial convenience and by technical ineffectiveness.

Another solution proposed by the prior art consists in limiting theenrichment on K₂CO₃ by the cells at the negative pole. The way ofobtaining it is disclosed in the U.S. Pat. No. 4,591,538. In the binaryelectrolyte Li₂CO₃/K₂CO₃ if the molar fraction of Li₂CO₃ is 72%, theions Li+ and K+ have the same mobility; therefore, in order to preventthe migration process a molar fraction of Li₂CO₃ comprised between 70%and 73% is claimed here.

But such a solution, in absence of complementary tools, remains totallyineffective against the drying\flooding phenomena of the extreme cells.

A further way for limiting the electrolyte migration withoutcompromising the gas exchange and the current transport is proposed inthe U.S. Pat. No. 4,761,348 by Kunz.

The solution consists in the combination of three elements:

-   -   a first porous layer (reservoir) at the negative pole of the        stack with at least a portion of its lateral face exposed to the        oxidant gas and in communication through the electrolyte with        the gasket, separated from the last cell by means of a        conductive plate which is impermeable for the electrolyte;    -   a second porous layer (reservoir) at the positive pole of the        stack, with at least a portion of its lateral face exposed to        the fuel gas and in communication through the electrolyte with        the gasket and which is separated from the first cell by means        of a conductive plate which is impermeable for the electrolyte;    -   the use between the manifold and the most external plate of a        gasket which is thicker than the one used between manifold and        cells.

The evident limit of this patent lays on the fact that in both thereservoirs the gas access is allowed only through the porosity of thelayers which form the reservoir itself and the performance of thecomplete process is therefore strongly diminished.

A way to avoid such a limit is disclosed by the patent U.S. Pat. No.5,019,464 of Mitsada and others, which describes the combination of:

-   -   at the positive side, exposed to the fuel manifold, a reservoir        constituted by one or more anodes and endowed with a current        collector\gas distributor    -   at the negative side, exposed to the oxidant manifold, a        reservoir constituted by one or more cathodes and endowed with a        current collector\gas distributor

The main limit of the solution described by such a patent is related tothe ohmic losses generated by the stack current through the cathodes ofthe “negative” reservoir. Usually such penalties become greater than thebenefit for the cell area of interest for commercial applications.

SCOPE OF THE INVENTION

Scope of the present invention is therefore to find a convenient way tocontrol the electrolyte migration process in large area MCFC stackswithout incurring in the disadvantages of the prior art.

The solution to this problem is to endow the molten carbonate fuel cells(MCFCs) stack with an electrolyte management tool, based on a slightlydifferent combination of external reservoirs, which however provides aradically different approach to compensate the electrolyte migrationprocess.

Namely also such a tool is based on a combination of two reservoir, onefor each side of the stack, both separated from the active cells by anelectronically conductive material which is impervious to gas; but a keyfeature of such innovative solution is that both are exposed exclusivelyto fuel gas environment and inaccessible to the oxidant gas.

More specifically such electrolyte management tool is characterized bythe combination of the following elements:

-   -   a positive reservoir component, external to the cathode of the        first cell on the positive side of the battery, wherein said        reservoir consists of one or more porous layers of        electronically conductive material and comprises at least one        gas distributor and    -   a negative reservoir component, external to the anode of the        last cell on the negative side of the battery, wherein said        reservoir consists of one or more porous layers of        electronically conductive material        reservoirs both exposed exclusively to fuel gas environment and        inaccessible to the oxidant gases.

Electronically conductive material impervious to gas separatesrespectively the positive reservoir from the cathode of the first cellon the positive side of the stack and the negative reservoir from theanode of the last cell on the negative side of the stack. The positivereservoir element is accessible to gases at least on one of the facesformed by the lateral surfaces of the cells, in which fuel gas ispresent and is separated from oxidant gases. In other words if, forinstance, the positive reservoir is exposed to the fuel inlet zone, theother three faces formed by the lateral surfaces of the cells areexposed respectively

-   -   to the oxidant gas fed to the stack,    -   to an exhausted oxidant gas outlet zone and    -   to an exhausted fuel gas outlet zone.

Namely, as standard for externally manifolded stacks, on every face, thegas is contained in a zone which is separated from the externalenvironment by means of gaskets attached to the perimeter of the faceand some parts of said gaskets are in contact with portions of the cellsmatrix.

The positive and the negative reservoirs too are in communicationthrough the electrolyte with gaskets which are in contact with thematrixes of the cells. Both the porous layers of the positive and of thenegative reservoir comprise at least 50 wt % of Ni; in particular,additionally, in one or in both the reservoirs those layers can furthercomprise elements consisting of anodes which are identical to the onesof the cells.

Porous gaskets are compressed on the perimeter of each of the 4 lateralfaces and thus the pore volume originally available is significantlydecreased. In a preferred embodiment, inside the strips which connectthe matrix of the first cell at the positive pole to the matrix of thelast cell at the negative pole, the volume of the residual porosity is<4% Such porosity is available to electrolyte for migration. Every cellof the stack comprises an anode, an electronically conductive fuel gasdistributor, a cathode, an electronically conductive oxidant gasdistributor and an electrolyte containing matrix. Preferably, the numberof the cells is >50 and their area is >3500 cm².

Two key features of the innovative combination are:

-   -   positive reservoir in fuel environment and endowed with gas        distributor    -   negative reservoir exclusively in fuel environment (irrelevant        if endowed with gas distributor or not)

Such a combination, fully effective against the drying of the cells nearto the positive pole, renounces to fully prevent the flooding of the“last” cell at the negative end of the stack. Its aim is to “stabilize”the flooding on a limit value and consequently to fully prevent anyextension of flooding to the other cells.

The key advantages are the fully negligible ohmic losses across thereservoirs. For stacks of large surface cells such advantage, in thewhole lifetime of the stack, is higher than the penalty of performancearising from the intrinsically delayed (and partial) flooding of onealone cell.

Namely, to decrease the emptying rate of the “first” one cell at thepositive side, is necessary to make competitive the positive reservoiras “source” of positive ions for the gaskets.

This means that the reactions with the gas to eliminate the CO₃ ²⁻ ionmust to can occur without penalties inside the positive reservoir, incomparison with the alternative inside the “first” one cell. This ispossible only in presence of two concomitant factors:

-   -   positive reservoir operating in fuel environment    -   positive reservoir endowed with a gas distributor.

Symmetrically, to decrease the flooding rate of the “last” one cell atthe negative side, is necessary to make competitive the negativereservoir as “collector” of positive ions from the gaskets.

This means to promote the forming of the CO3=inside the negativereservoir. This is possible only in presence of two concomitant factors:

-   -   negative reservoir operating in cathodic environment    -   negative reservoir endowed with a gas distributor.

The ohmic losses in cathodic environment are quite high, in fuelenvironment negligible; therefore it's evident that any effort to fullyprevent the flooding of the cells at the negative end by a “cathodic”reservoir introduces ohmic losses.

Instead the ohmic falls produced by the passage of the stack currentthrough an “extra-anodes” reservoir (i.e. a reservoir exposedexclusively to fuel environment) are negligible and stable with time.

For instance a positive reservoir split in sections, each constituted by2 extra-anodes and 1 one collector/distributor, shows ohmic losses inthe range of 1-2 mV across each section.

The volume available to store electrolyte inside the positive reservoircan be increased, to meet increased life target, simply by increasingthe number of conductive porous layers and eventually also of gasdistributors inside the reservoir. A set of one current collectors andrelated porous layers is a “section” of the reservoir.

Different sections are separated by metallic sheets. By endowing thepositive reservoir with many sections, the delivery of positive ions tothe gasket occurs at the same speed in every section. Namely so lowvoltage drops across the sections do not produce sufficient counterfields to affect the efficacy of the farthest sections, so that even fora positive reservoir with 5-10 sections all concurs with comparableeffectiveness to the cell protection.

By changing the gasket, the electrolyte quantity transferred in aparticular time interval from the positive side of the stack to thenegative one changes. But when using the same gasket this quantity doesnot depend either on the cells number or on the cells area. Theinfluence of the migration effect for a particular cell, on thecontrary, depends directly on the cell surface: for small-area cells inbrief time laps the electrolyte quantity transferred is already relevantrelatively to the carbonates content of the cell. For “commercial” cellsof large area, instead, many thousands hours are necessary before theeffect can be noted.

By means of the MCFC apparatus of the present invention, it is possibleto avoid the anticipated emptying of the cells at the positive pole,which could cause a power drop and the mixing of the anodic and cathodicgases, what generates devastating hardware degeneration.

The protection of positive cells with a reservoir of extra anodesendowed with the collector of current\gas distributor, allows to reach40,000 hours of working time with a power loss which for a stack of 125kW is minor of 0.02 percent of the power in the entire “working life”.

By using a reservoir of extra-anodes even at the negative pole, as it isexposed exclusively to fuel environment, under operating conditions theLi and K ions from the gasket flow in the last negative cell, becausethe formation of CO₃ ²⁻ occurs more easily on the cathode of the cellthan in the reservoir. Therefore the mechanism which active the floodingof the last “negative” cell is not avoided. Nevertheless, the negativereservoir is in communication through the electrolyte with the last celland both the cell and the reservoir, which is in fuel environment, areat the same voltage. In those conditions, together with the gradualflooding of the cell, the gradual filling of always bigger pores by theelectrolyte, do active the driving force for a capillary transfer ofelectrolyte into the large volume of smaller, free pores available inthe reservoir.

With this process, the systematic removal of electrolyte from the lastcell generates a dynamic flow which limit the flooding level of the most“negative” cell and, consequently, leads to the effective “protection”of all the other cells. The protection times can be easily extended bychanging the number of extra-anodes in the negative reservoir whichgives a larger volume for receiving a greater quantity of electrolyte.In the negative reservoir, as the electrolyte is collected by capillaryforces, the presence of a gas distributor is not indispensable.

The ohmic losses are active in the whole lifetime of the stack and areincreasing proportionally to the reservoir size. Instead thepolarization losses, caused by flooding, are active only after that theflooding is started. For large area cells, no flooding occurs, neitherin the most “negative” cell, for at least about 10,000hours. With theproposed solution, ohmic losses remains negligible even by increasingthe reservoir size; only one cell is affected by polarization losses dueto flooding and moreover its flooding remains incomplete, as oncereached a threshold, the protection mechanism of the negative reservoiris activated.

Therefore, the use of extra anodes results to be a solution which is inany case technically and economically convenient; additionally, itallows to reach an absolutely irrelevant margin of power loss during theentire operating time of the stack.

DESCRIPTIONS OF THE DRAWINGS

The following embodiments of the invention have a pure explanatorynature and should be therefore interpreted without any restriction tothe general inventive concept.

FIG. 1 shows an overview of the components disclosed in the presentinvention.

FIG. 2 shows a possible embodiment of the reservoir at the positive sideof the stack.

FIG. 3 shows a possible embodiment of the reservoir at the negative sideof the stack.

Referring to the embodiment shown in FIG. 2, the reservoir is locatedbetween the positive plate 13 and the cathode of the first cell at thepositive side, from which it is separated by means of a separating plate14 which is of the same type of the plates used to separate the cells inthe stack. Also the current collector\gas distributor 15, the cathode 16and the matrix 18 of the first one cell are shown.

In the above example the positive reservoir is divided in two sections,where each one is constituted of two extra-anodes 11 and a currentcollector/gas distributor 12. The two sections are separated by themonopolar plate 17. If the sections in the reservoir are more than two,all the internal sections are separated by means of a monopolar platesimilar to 17. The separation from the cathodic gas can be obtainedthrough the matrix strips 19 filled by electrolyte.

Analogous to FIG. 2, FIG. 3 shows a possible embodiment of the reservoirat the negative side of the stack. This reservoir is inserted betweenthe terminal negative plate 23 and the anode of the last cell at thenegative side, from which it is separated by means of a separating plate24. The current collector/gas distributor 25, the anode 26 and thematrix 28 of the last cell of the stack are also shown.

In the above example the positive reservoir is divided in two sections,where each one is constituted of two extra-anodes 21 and a currentcollector/gas distributor 22. The two sections are separated by themonopolar plate 24. The separation from the cathodic gas can be obtainedthrough the matrix strips 29 filled by electrolyte.

If the sections in the reservoir are more than two, the internalsections are separated by means of a monopolar plate similar to 24.

In this reservoir, the current collector\gas distributor can beeliminated.

The invention claimed is:
 1. A Molten Carbonate Fuel Cell stackcomprising a plurality of cells separated by an electronicallyconductive material which is impervious to gas, characterized by thecombination of the following elements: a positive reservoir component,external to the cathode of the first cell, on the positive side of thestack, wherein said reservoir consists of one or more porous layers ofelectronically conductive material and comprises at least one gasdistributor and a negative reservoir component, external to the anode ofthe last cell, on the negative side of the stack, wherein said reservoirconsists of one or more porous layers of electronically conductivematerial, wherein both the reservoirs are in use exposed exclusively tofuel gas environment and are inaccessible to the oxidant gases, all theporous layers of all the reservoir components being inaccessible to theoxidant gases.
 2. Molten Carbonate Fuel Cell stack as defined in claim1, wherein the positive reservoir is separated from the cathode of thefirst cell on the positive side of the stack by means of anelectronically conductive material which is impervious to gas.
 3. MoltenCarbonate Fuel Cell stack as defined in claim 1, wherein the negativereservoir is separated from the anode of the last cell on the negativeside of the stack by means of an electronically conductive materialwhich is impervious to gas.
 4. Molten Carbonate Fuel Cell stack asdefined in claim 1 or 2, wherein the positive reservoir element is inuse accessible to gases at least on one of the faces formed by thelateral surfaces of the cells, in which fuel gas is present and which isseparated from oxidant gases.
 5. Molten Carbonate Fuel Cell stackaccording to claim 1, wherein the positive and the negative reservoirsare in use in communication through the electrolyte with gaskets whichare in contact with the matrixes of the cells.
 6. Molten Carbonate FuelCell stack according to claim 1, wherein the positive and the negativereservoir are in use accessible to the fuel gas at the fuel inlet sidewhile the other three faces formed by the lateral surfaces of the cellsare in use exposed respectively to the oxidant gas fed to the stack, toan exhausted oxidant gas outlet zone and to an exhausted fuel gas outletzone.
 7. Molten Carbonate Fuel Cell stack according to claim 1, whereinthe positive and the negative reservoir are in use accessible to thefuel gas at the exhaust fuel outlet side while the other three facesformed by the lateral surfaces of the cells are in use exposedrespectively to the oxidant gas fed to the stack, to an exhaustedoxidant gas outlet zone and to the fuel gas fed to the stack.
 8. MoltenCarbonate Fuel Cell stack according to claim 6, wherein gaskets areattached to the perimeter of the face, some parts of said gaskets are incontact with portions of the cells matrix and wherein in use on everyface of the cell stack the gas is contained in a zone which is separatedfrom the external environment.
 9. Molten Carbonate Fuel Cell stackaccording to claim 1, wherein every cell of the stack comprises ananode, an electronically conductive fuel gas distributor, a cathode, anelectronically conductive oxidant gas distributor and an electrolytecontaining matrix.
 10. Molten Carbonate Fuel Cell stack according toclaim 1, wherein in porous gaskets compressed on the perimeter of thefaces, the portions which connect the matrix of the first cell at thepositive pole to the matrix of the last cell at the negative pole, havea volume of residual porosity in the gaskets which is <4%.
 11. MoltenCarbonate Fuel Cell stack according to claim 1, wherein the porouslayers of the positive and of the negative reservoir comprise at least50 wt% of Ni.
 12. Molten Carbonate Fuel Cell stack according to claim 1,wherein in the porous layers of the positive and of the negativereservoir at least 50 wt % is Cu or Ni+Cu.
 13. Molten Carbonate FuelCell stack according to claim 12, wherein the porous layers of thenegative reservoir comprise elements consisting of anodes which areidentical to the ones of the cells.
 14. Molten Carbonate Fuel Cell stackaccording to claim 13, wherein the porous layers of the negativereservoir comprise elements consisting of anodes which are identical tothe ones of the cells.
 15. Molten Carbonate Fuel Cell stack according toclaim 1, wherein the number of the cells is >50 and their area is >3500cm².
 16. A Molten Carbonate Fuel Cell stack comprising a plurality ofcells separated by an electronically conductive material which isimpervious to gas, characterized by the combination of the followingelements: a positive reservoir component, external to the cathode of thefirst cell, on the positive side of the stack, wherein said reservoirincludes one or more porous layers of electronically conductive materialand comprises at least one gas distributor; and a negative reservoircomponent, external to the anode of the last cell, on the negative sideof the stack, wherein said reservoir includes one or more porous layersof electronically conductive material; wherein both the reservoirs arein use exposed exclusively to fuel gas environment and are inaccessibleto the oxidant gases, all the porous layers of all the reservoircomponents being inaccessible to the oxidant gases.